In recent years, N-heterocyclic carbenes
(NHCs)
have garnered significant attention as promising alternatives to thiols
to stabilize metallic nanoparticles and planar surfaces. While most
studies thus far have focused on NHC-functionalized gold nanoparticles
(AuNPs), as an ideal platform to investigate the role of NHCs in stabilizing
such nanoparticles, their ability to protect more unstable coinage
metal nanoparticles, such as silver nanoparticles (AgNPs), has been
largely overlooked. This is despite the fact that AgNPs possess a
much more sensitive optical response that, upon their enhanced stability,
can broaden their scope of application in various fields, including
nanomedicine and catalysis. In this study, the synthesis and use of
monomeric and polymeric mesoionic NHC–Ag(I) complexes as precursors
to mono- and multidentate NHC-tethered AgNPs are reported. The polymeric
analog was obtained by first synthesizing a polymer, containing 1,2,3-triazole
repeat units, employing the copper-catalyzed alkyne–azide cycloaddition
click polymerization of monomers containing diazide- and dialkyne
functional groups. Subsequent quaternization of the triazole moieties
and Ag insertion yielded the target NHC–Ag-containing polymer.
Using this polymer as well as its monomeric analog as substrates,
AgNPs with either catenated networks of NHCs or monomeric NHCs were
fabricated by their reduction using borane–tert-butylamine complex. Our stability studies demonstrate that while
monomeric NHCs impart some degree of stability to AgNPs, particularly
at elevated temperatures in aqueous as well as organic medium, their
polymeric analogs further enhance their stability in acidic environment
(pH = 2) and against glutathione (3 mM), as an example of a biologically
relevant thiol, in aqueous media. To highlight the application of
these NHC-functionalized AgNPs in catalysis, we explore the aqueous
phase reduction of methyl orange and 4-nitrophenol.